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SubjectRe: [PATCH v3 6/7] thermal/drivers/cpu_cooling: Introduce the cpu idle cooling driver
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On 05/08/2019 07:11, Martin Kepplinger wrote:
> ---

[ ... ]

>> +static s64 cpuidle_cooling_runtime(struct cpuidle_cooling_device *idle_cdev)
>> +{
>> + s64 next_wakeup;
>> + unsigned long state = idle_cdev->state;
>> +
>> + /*
>> + * The function should not be called when there is no
>> + * mitigation because:
>> + * - that does not make sense
>> + * - we end up with a division by zero
>> + */
>> + if (!state)
>> + return 0;
>> +
>> + next_wakeup = (s64)((idle_cdev->idle_cycle * 100) / state) -
>> + idle_cdev->idle_cycle;
>> +
>> + return next_wakeup * NSEC_PER_USEC;
>> +}
>> +
>
> There is a bug in your calculation formula here when "state" becomes 100.
> You return 0 for the injection rate, which is the same as "rate" being 0,
> which is dangerous. You stop cooling when it's most necessary :)

Right, thanks for spotting this.

> I'm not sure how much sense really being 100% idle makes, so I, when testing
> this, just say if (state == 100) { state = 99 }. Anyways, just don't return 0.
>
> Daniel, thanks a lot for these additions! Could you send an update of this?

Yes, I'm working on a new version.

> btw, that's what I'm referring to:
> https://lore.kernel.org/linux-pm/1522945005-7165-1-git-send-email-daniel.lezcano@linaro.org/
> I know it's a little old already, but it seems like there hasn't been any
> equivalent solution in the meantime, has it?
>
> Using cpuidle for cooling is way more effective than cpufreq (which often
> hardly is).

On which platform that happens?


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